Various methods for swaging needles to sutures are known. In a surgical needle of a well-known type, the free end of a suture is inserted into an axial bore within a blunt end of the needle (the “needle barrel”), and held in that position while a swage die impinges upon the outer surface of the needle barrel, thereby compressing a portion of the bore onto the suture. The compressed portion of the bore grasps the suture by mechanical interference and by surface contact (friction). The swaging process is conducted so as to create an attachment between the needle barrel and suture that meets or exceeds the attachment (or “pull-out”) strength requirements of standards such as USP 23 <871>.
One approach to providing good suture attachment is multiple hit swaging, wherein a needle is subjected to swaging (i.e., compression) of controlled depth, but distributed over a large area (e.g., around the circumference of the needle). To achieve this type of swaging, the needle may be rotated relative to the swaging dies between multiple swaging compressions. In this manner, multiple angularly offset swaging operations (hits) are performed to attach a single needle to a single suture. While this approach provides good attachment, each hit on the barrel of the needle produces stress in the needle barrel and suture. The materials of which the needle and suture are made inherently have some degree of malleability, but when the limit of malleability is reached, the material will fail, leading to, in the case of the needle, cracking and loss of attachment, or breakage. Cracking is a particular problem when harder alloys are used, including advanced alloys such as 4310 SS, nickel-titanium SS, and 420 SS. Further, needle materials have some elasticity, such that the relief of residual stress causes the needle barrel to relax over time, leading to a loss of attachment between the needle bore and the suture.